1. Technical Field
The present invention relates generally to the field of audio recording and playback systems. More particularly, it relates to the field of digital audio recording and playback systems.
2. Background Art
Typical audio recording systems known in the art use magnetic tape, drums or disks as storage media for storing audio signals for later playback. An audio sensor or detects analog audio signals which are converted to electronic pulses. The storage medium typically is mechanically rotated to expose an unwritten section thereof to an electronic unit which records the incoming signal on the medium.
Such systems suffer from various disadvantages. For example, the mechanical parts of the system may wear, especially those parts which rotate or otherwise move. The media themselves may be damaged in normal use, as when a tape is stretched or a disk is scratched, thereby decreasing the quality of the recording and playback. Moreover, the mechanical parts of those systems usually require periodic maintenance or replacement. Such systems also suffer from a limitation on the ability of the user to select randomly a portion of the recording to be played. That is, the messages must be accessed serially, as, for example, by rewinding the recording tape. And the portion of the recording desired may be difficult to locate readily, resulting in excessive search and effort.
3. Summary of the Invention
The present invention is an improvement of the digital recorder/player of U.S. Pat. No. 4,772,873 to the present inventor, which is incorporated herein by reference. The present invention is a modular system for digital random access recording and playback of audio signals. A single module may comprise one logic board, and up to sixteen channels. Each of the channels may have up to sixteen memory boards per channel. A power supply and interconnect cables also are provided. An entire system may comprise several modules. In a system with four modules, for example, there could be four independent audio sources, a crystal controlled master oscillator to provide uniform clocking to the modules, four logic boards, up to 1,024 memory boards, a power supply and interconnect cables. A timer may also be provided for timed random access recording and playback.
Preferably, according to the present invention, an audio signal is picked up and filtered by an analog filter which blocks DC and very low frequencies. Then the signal is filtered by digital filters to prevent foldover distortion of the analog audio signal. One of the digital filters also preferably generates a clock, although it can be slaved to an external clock. The other filters and an analog to digital/digital to analog converter (ADC/DAC) are slaved to the digital filter clock to prevent harmonic heterodyning. A master oscillator may be provided in a system incorporating multiple modules.
Preferably, the ADC/DAC is a continuous variable slope delta (CVSD) modulator system which samples the analog audio signal at a uniform frequency. The digital signal output of the ADC is fed to a memory board. The clocking for the CVSD and for the logic board is derived, through a divide-by-eight counter, from the digital filter clock. The logic board controls one module of the system and serves as the controller for the memory boards, an encoder for analog to digital conversion, a decoder for digital to analog conversion, an interface for the system and module controls, and provides clocking for each individual module.
The memory of each module of the system of the present invention preferably includes up to sixteen channels, each having up to sixteen memory boards. The memory technology may include CMOS static RAMs, EPROMs or EEPROMs. To accomplish random accessing, each of the channels is addressed by a unique four bit binary code and each of the memory boards is addressed by a unique four bit binary code. Thus, a complete address for a particular memory board of a particular channel is an eight bit code including the unique channel code and the unique board code. Each board of a particular channel will have the same four bits of channel code and a different four bit board code.
The logic board communicates with the memory of the system via a memory buss which includes a 34 line edge pin connector which connects with all memory boards of the module. The logic board includes a "D" flip-flop to define the reset and play modes. When power is first applied to the logic board, the logic board resets. Random access for the recording and play modes is achieved by each memory board using two four bit comparators, one which compares the four bit channel code on the memory buss with the setting of a four bit switch, and one which compares the four bit board code on the buss with the four bit switch on the memory board itself. An external control switch selects the record or play mode and selects a channel and memory board to receive recorded data or from which data are played back. When there is a match of codes, the memory control logic on an individual memory board is enabled and data is written to or retrieved from a particular memory board.
The logic board is also provided with channel and memory board counters for the sequential use of the channels and memory boards in a module. That is, the first memory board of the first channel is filled with messages first, then the second memory board of the first channel, and so on through the sixteenth memory board of the first channel. Then the second channel is enabled, and each of its memory boards is filled sequentially. The process continues until all 256 memory boards of the sixteen channels are filled. Alternatively, the present invention provides for external control of the channel and memory board selection to provide random access to the memory.
During the record cycle, a selected memory chip is enabled to read incoming data, and data are passed in serial format from the logic board to the selected memory board to be converted to parallel format by a serial to parallel converter, and data are strobed into memory.
During the playback cycle, the control logic on the memory board selects a memory chip, the data corresponding to an address supplied by address counters. The data are converted from parallel to serial format by a converter and are transmitted back to the logic board for conversion from digital to analog signals. The analog signal is filtered and amplified before passing to an audio signal output device such as a loudspeaker.
By providing for digitalization of an audio signal and storage of the signal in digital form in a memory and by providing for random access of the signals so stored, the present invention overcomes disadvantages and limitations of audio recording systems of the past.
Accordingly, it is a primary object of the present invention to provide an audio recording and playback system which converts an audio signal to digital form and stores the digital signal in a memory such that random access for recording and playback is achieved.
It is another object of the present invention to provide a digital audio recording and playback system with a memory unit having a plurality of channels, each channel having a plurality of memory boards, and in which channels and memory boards can be randomly accessed.
These and other objects, features and advantages of the present invention are described in or are apparent from the following detailed description of a preferred embodiment.